High resolution digital cameras have been developed for professional photographers, whose business depends on taking high quality pictures. These cameras often use existing 35 mm or medium-format film camera bodies and lenses, together with a customized electronic imaging back that replaces the removable film back ordinarily supplied with the camera. An example is the Kodak DCS 460 camera, which is based upon the Nikon N90s single lens reflex (SLR) camera body. This system, which is compatible with all Nikon lenses and accessories, allows photographers to easily operate the digital camera, and to use their existing equipment with the camera. However, the photosensitive area of the imager is often smaller than the film area, and therefore smaller than the image plane. As a result, the relative magnification of the lens is larger than for 35 mm film.
A problem with this type of camera is the alignment of the image sensor to the optical axis of the camera body. For a non-SLR camera, where the sensor may be an integral part of an optical assembly, the alignment technique in Japanese patent application 61-245773 can be used. Two positioning pins are provided to the rear end of a lens barrel, and are pressed into contact with a ceramic package containing the image sensor in order to optically position the image sensor at an image plane of a lens assembly. However, in an SLR camera system, the image sensor cannot physically locate to the lens assembly because the lens, which is removable, is separated from the image plane by a movable mirror used for through-the-lens viewing.
Consequently, in a camera back system using a film body, the sensor needs to be referenced to the film plane. For example, as shown in U.S. Pat. No. 5,483,284, a CCD package possesses four positioning pins at four corners thereof that touch a guide rail plane of an aperture in the camera body. With this arrangement, the CCD package is precisely located at the focal plane of the lens assembly, i.e., in the z dimension. However, it is difficult to prevent x-y misalignment, which in turn prevents accurate centering of the CCD package relative to the optical axis of the lens assembly.
Because the photosensitive area of the imager is often smaller than the optical image plane in the camera, which is designed for 35 mm film, the viewfinder image area likewise needs to be reduced in size to match the imager area. This is ordinarily done by masking over the non-captured region of the viewfinder image. Customers of professional digital back cameras, however, are demanding improved accuracy in representation of the captured image center relative to the camera viewfinder. The above-mentioned '284 patent does not provide accurate centering of the image sensor relative to the optical axis of the camera. What is needed is a reliable method for optical centering of the imager in an SLR system.